Solicitor General Donald B. Verrilli, plans to file a brief today with the Supreme Court in favor of challengers of Prop. 8, according to an administration source.

It would mark the first time that the Obama administration has come out in court against the California ballot initiative that defines marriage as between one man and one woman.

As far back as 2008, the president said that he thought Prop 8 was "divisive and discriminatory," but his Justice Department has never opined on its constitutionality. Because the DOJ is not a party to the case, it is not required to file a "friend of the court" brief, but the deadlines for briefs supporting the challengers to Prop 8 is tonight at midnight.

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Theodore Olson, one of the lead lawyers challenging Prop 8, told reporters last week that he hoped the DOJ lawyers would take the opportunity to set down a legal position.

In Depth: Obama's Prop 8 Decision

"However," Olson added, "whether they do or not, the president of the United States made it very clear in his inaugural address that we cannot rest in America until all civilians have equal rights under the law so, in a sense, the president has made that statement already."

Today, 39 states have laws restricting marriage to opposite-sex couples. This number includes voter-approved constitutional amendments in 30 states barring same sex marriage. Nine states allow gay marriage.